“Shine on… Shine on Harvest Moon… Up in the sky. I ain’t had no clear skies since January, February, June and July…” Oh! Greetings, fellow SkyWatchers! Welcome to this week’s edition where we’ll be taking a closer look a lunar features and just what makes a globular… Well… A globular! Time to get out the binoculars and telescopes and turn a eye to the sky, because…
Here’s what’s up!
Monday, October 2 – Tonight on the lunar surface, we’ll return to previous study Eratosthenes. Located on the south shore of Mare Imbrium right where the Apennine mountain range meets the terminator, Eratosthenes is one unmistakable crater. Named after the ancient mathematician, geographer and astronomer Eratosthenes, this splendid Class 1 crater is 58 km in diameter and 12,300 feet deep. Tonight it will display a bright west wall and a black interior hiding a massive crater-capped central peak that reaches 3570 meters high. Extending like a tail, a 50 mile long mountain ridge angles away from Eratosthenes to the southwest.
Now let’s dance. If you thought Eratosthenes was it, power up and look again. Just at the end of that southwest trail of mountains are the ruins of crater Stadius, which is peppered with small meteor impacts. Do you remember Shoemaker-Levy and Jupiter? Then look to Stadius’ northwest where you will see a long line of impact craters that must have occurred at roughly the same time from a series of similar sized meteors. If you turn east through Sinus Aestuum, you might spot the small impact of Bode. Go south of Stadius and trace the rilles through Mare Insularum to the blank, small ring of Gambart. Just northeast of this crater are two small punctures and the landing area of Surveyor 2.
Now let’s go back to Lacerta and have a look at another moderately bright open cluster – NGC 7209. This large, 6.7 magnitude open cluster is normally visible in binoculars as a faint blur. Dominated by a handful of scattered brighter stars, most of this cluster’s fainter members take up residence in their midst. Start at Pi 2 Cygni and head a bit more than two finger-widths southeast. If you get to a line of fifth magnitude stars running northeast-southwest you’ve gone too far – but in the right direction!
Tuesday, October 3 – We start tonight’s lunar tour with something that can even be spotted with unaided vision – Plato. Located in the northern hemisphere of the Moon, its dark ellipse is unmistakable. Plato’s floor consists of 2700 square miles of lava fill and is considered by some observers as the darkest single low-albedo feature on the Moon. Because of its low reflectivity, this crater has the distinction of being one of the only mountain-walled plains that doesn’t “disappear” as the Moon grows full. With Plato in the center of the field note the pyramid-like peak of Pico due south in northeastern Mare Imbrium. East of Pico is an unnamed dorsum – or lava wave – terminating just above crater Piazzi Smyth to the south. Power up and check out the triangular peak near its end.
Having looked at the Moon tonight, take the time to view bright southern star Fomalhaut ascending low to the southeast. Also known as “The Lonely One,” Alpha Pisces Austrinus resides in a rather desolate area of the southern sky some 23 light-years away. At magnitude 1.3, Fomalhaut is the 18th brightest star of night sky. This star is nearer, but one magnitude fainter than Vega – a star of similar spectral type. At twice the diameter of our Sun, “The Lonely One” is 14 times more luminous than Sol and is surrounded by what could be a protoplanetary accretion disk.
Wednesday, October 4 – Today in 1957, Sputnik 1 made space history by becoming the first manmade object to orbit the earth. Our Earth’s first artificial satellite was tiny – roughly the size of a basketball – and weighed no more than the average man. Its very first 98 minute elliptical swing around the Earth set off a “space race” that inspired man to the Moon. Many of us old enough to remember Sputnik’s grand passes recall just how inspiring it truly was. Take the time with children or grandchildren to check heavens-above.com to learn about the visible passes of the ISS and other bright satellites as you think about how spaceflight has changed over the last 50 years!
Tonight on the lunar surface, look southwest of striking crater Bullialdus for a pair of similar sized craters on the shore of Mare Nubium – Mercator to the southeast and Campanus to the northwest. Just to their south you will see a triangular dark area that looks like it might be part of Mare Nubium, yet has a few bright points of its own. This is Palus Epidemiarum, a very small plain. Look for the oval of crater Capuanus trapped on its southern edge.
Now drop back the power and look at the field of stars around the Moon…you just might discover that one of them is a planet! Not only will Uranus be very close tonight, but there will also be an occultation for some areas, so be sure to check IOTA for information.
For southern hemisphere viewers, tonight would be a wonderful opportunity to re-discover one of the finest double stars in the sky – Rigel Kentaurus. Located low to the southwest, Alpha Centauri is the third brightest star in the sky, yet the most famous due to its distance of 4.34 light-years.
Thursday, October 5 – Today marks the birth date of Robert Goddard. Born in 1882, Goddard is known as the father of modern rocketry – and with good reason.
In 1907, Goddard came into the public eye behind a cloud of smoke rising from the thruster of a small solid-fuel rocket in the basement of the physics building of Worcester Polytechnic Institute. By 1914, Robert had patented the use of liquid fuel and multi-stage solid fuel rockets. He was driven by efforts to put equipment ever higher, and by 1920 Goddard had envisioned rockets reaching the Moon. Among his many achievements, he proved that a rocket would work in a vacuum and by 1926 he had sent the first scientific package along for the ride. By 1932, Goddard launched guided flights and in 1937 mounted thrusters on gimbals and stabilized them gyroscopically. Goddard’s lifetime of work went pretty much unnoticed until the dawn of the Space Age, but in 1959 (14 years after his death) he received due acclaim as NASA’s Goddard Space Flight Center was established in his memory.
Today in 1923, Edwin Hubble was busily discovering the first Cepheid variable in M31 – the Andromeda Galaxy. Hubble’s discovery was crucial in proving that “spiral nebulae” were actually independent and external “island universes” similar to our own Milky Way Galaxy.
Tonight let’s look at a Cepheid variable as we head towards… well… Cepheus! Let’s visit the very Delta Cephei that is the prototype for all such variables. Easily found as the easternmost in a trio of stars marking the southeastern corner of the constellation, “3.9” magnitude Delta, is the great-granddaddy of all stars with smooth and predictable changes in luminosity. In fact, this one is so predictable that you could set your “5 day, 8 hour, 47 minute, and 32 second” clock to it. Never wandering over a range more than 0.8 magnitudes – you won’t see this one disappear like Mira in Cetus. As Delta’s luminosity changes, so does its photospheric temperature and spectral class. Ranging from spectral class “F” (6800 degrees Kelvin) to class “G” (5500 degrees) this supergiant swells and contracts rhythmically in size. Located around 300 light-years away, Delta never gets as faint as our Sun would be at 1/10th the same distance. It is because of great luminosities, and predictable behaviors, that Cepheid variables have become the “yardstick” of the Universe. All you have to know is how long the cycle of a Cepheid variable is and you also know how bright they are and with that knowledge you can figure out the distance. Ranging from a day to nearly two months in periodicity, longer cycles mean brighter stars. Find one in some relatively nearby galaxy and all the guesswork is gone!
Be sure to look for a 6.3 magnitude companion to Delta Cephei while you’re there…
Friday, October 6 – For those in western time zones, tonight is one of the most famous Full Moon nights of the year – the Harvest Moon!
Through the magic of Universal Time (UT), the Moon is full in the Americas during early evening hours of the previous day and because of this will be the closest full moon to the Autumnal Equinox. Not only is it at its closest right now, but the Moon’s orbit is almost parallel to the eastern horizon causing it to rise at dusk for the next several nights in a row. Normally, the Moon clears the horizon some 50 minutes later each night, but at this time of year, the delay is just 20 minutes for mid-northern latitudes and even shorter further north. Because of this added extra light, the name “Harvest Moon” originated because it allowed farmers more time to work in the fields.
Oftentimes we perceive the Harvest Moon as being more orange than at any other time of the year. The reason is not only scientific enough – but true. Coloration is caused by the scattering of the light by particles in our atmosphere. When the Moon is low, as it is now, we get more scattering effect and it truly is a deeper orange. The very act of harvesting itself also produces dust and oftentimes that color will last through the whole night.
So, cursing the Moon for lighting the sky tonight, enjoy it for what it is…a wonderful, natural phenomenon!
Saturday, October 7 – Today celebrates the birthday of Niels Bohr. Born in 1885, Bohr pioneered atomic physics. In his quest to understand atoms, Bohr had a vision – one right out of the night sky. He conceived of the atom as a tiny solar system, where electrons became planets and the nucleus a tiny sun!
Today is also the official UT date of the full moon. Tonight three planets grace the night sky at skydark. Furthest west is distant Pluto – less than one degree southeast of Xi Serpentis. Neptune has now progressed a bit more than one degree northwest of Iota Capricorni and Uranus is near Lambda Aquarii. If you rise early, you can also catch Saturn preceding Regulus across the sky in good observing position. Venus and Mars are now too close to the Sun for observation. Mercury and Jupiter are very low to the western horizon at sunset. And the Earth? You won’t see it as long as you keep looking up!
Sunday, October 8 – Today marks the birthday of Ejnar Hertzsprung. Born in 1873, Hertzsprung proved the existence of giant and dwarf stars in the early 1900s. His work implied a relationship between color and luminosity, but his method wasn’t truly recognized until rediscovered by Henry Russell. Later that method became the foundation for virtually all subsequent astronomical work as the Hertzsprung-Russell diagram. Hertzsprung’s use of absolute magnitude applied to that diagram will come into play tonight – and throughout the week – as we have a look at the globular cluster M15 in Pegasus.
One of the most profound questions of the early 20th century related to the true age and scale of the universe. To solve this riddle meant to get a sense of the nature of the stars. As astronomers measured more and more stellar distances (based on methods pioneered by Bessel), it became clear that stars varied tremendously in absolute brightness – not because of distance – but because of things like size, temperature, mass, and age. Most stars were found to be similar to our own Sol. Such stars continue to illuminate a very small part of the Milky Way galaxy for billions of years. The stability of our Sun – and many like it – meant they were living life in the middle lane – neither squandering nuclear fuel, nor hoarding it. These stars are found in the main sequence region of Hertzsprung and Russell’s (H-R) diagram and a have a predictable range of brightness based on color and surface temperature.
But, the very brightest stars don’t necessarily fall in line in this way. Some are very massive, youthful, and hot – such as Deneb. Others are very old, less massive, relatively cold, and hugely swollen – such as Antares. A very few are long or short term variables like Delta Cephei and RR Lyrae. These “standard candle” variables could be used to determine the scale of things in the early 1900s, but their distances still had to be worked out!
Tonight head about two finger-widths north-east of Epsilon Pegasi with either binoculars or scope. As you look at 6.4 magnitude globular cluster M15, you will see something that can supply the key to both the age and size of the then known universe – our Milky Way.
May all your journeys be at light speed… ~Tammy Plotner with Jeff Barbour.